Equatorial Guinea has been ruled with an iron fist by Teodoro Obiang Nguema since 1979, which makes him the longest-standing ruler on the continent.

Black Beach prison, built under Spanish rule in the 1940s, lies near a black sand beach in Malabo and its name alone sends chills down the spines of Equatoguineans.

Thousands of people are believed to have been tortured and killed there.

Dario Martinez alleged that Nzogo's arrest was politically motivated and said that the schoolmaster had been in contact with Daniel Lebegue, who chairs the French branch of Transparency International.

Lebegue's group was largely responsible for the launch of a French investigation into so-called "ill-gotten gains", an unprecedented case which has led to an arrest warrant against Obiang's son -- and vice-president -- Teodorin.

Earlier this month, Equatorial Guinea responded in kind by issuing an arrest warrant against Lebegue.

The French probe is seeking to establish whether several African presidents and their families have embezzled public funds to acquire vast assets in France.

In the Obiang family's case, those assets have been estimated at around $200 million, including a huge mansion on one of the most expensive streets in Paris which was raided last year.